EP0193889B1 - Process for regenerating inactive and poisened platinum catalysts - Google Patents

Description

It is known to produce hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen over platinum metal-containing catalysts in an acidic medium. The activity, selectivity and service life of the catalyst used are of essential importance for the technical feasibility of the process.

This process uses a supported platinum catalyst which, when used in the reactor, loses selectivity and activity noticeably over a period of a few weeks. The catalyst must therefore be regenerated periodically. For the service life of the catalyst, defined as the time in which the catalyst remains in operation continuously in the reactor, the process of the respective regeneration is also of great influence. DE-PS 1 088 037 describes a conventional regeneration and its effect on the subsequent driving periods.

Despite periodic regeneration, a catalyst that is used for a long time slowly loses its original catalytic properties; the selectivity drops and the downtimes become shorter. More frequent regeneration is required.

The cause of these long-term effects is essentially an increasing occupation of the catalyst with poisoning metals. These are transferred to the catalyst via the acidic reaction medium during the corrosion of line and boiler steel. During the regeneration, the metals can largely be removed with aqua regia, but when the platinum is reductively precipitated, they are again deposited on the catalyst support.

The previously known methods for cleaning the catalyst from these metal impurities failed because of the high technical outlay and the high platinum losses associated with the cleaning.

It was therefore the task of developing a process for restoring the original catalytic properties of long-used platinum supported catalysts which are used in the production of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in aqueous solutions of mineral acids.

The invention relates to a process for cleaning poisoned supported platinum catalysts, by means of which the disadvantages mentioned above are avoided.

Surprisingly, it has been found that noble metal catalysts, in particular platinum catalysts, which are supported and which have been poisoned by metal through many years of use in the production of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in aqueous mineral acid, can be regenerated without particular technical and apparatus expenditure , if the metallic impurities are dissolved with nitric acid or aqua regia, the solution is neutralized, the interfering metals fall out of the neutralized solution through a selective precipitation reagent and the purified platinum solution is reused for catalyst production.

If the platinum catalyst is heavily loaded with heavy metals, the metallic contaminants can be dissolved in aqua regia.

As selective precipitation reagents, one can advantageously use organic complexing agents which form water-insoluble or poorly soluble complexes with the metallic impurities, such as, for. B. quinoline, carbazone, aminohydroxybenzene and anthraquinone derivatives. The precious metal losses are limited to a minimum by the method according to the invention.

In general, a large part of the metal impurities go into solution in dilute nitric acid, the platinum remains essentially undissolved on the support; after neutralization of the filtered nitric acid solution, one can selectively precipitate the interfering metals contained therein in concentrated form, while the small amounts of platinum present remain in solution. Complexing agents which form complexes which are sparingly soluble with the interfering transition metals but not with platinum are suitable as selective precipitants. After the precipitation has been separated off, the filtrate can be slurried with the pre-cleaned catalyst in dilute aqua regia and processed further.

When treating with dilute nitric acid, the separation of interfering metals remains incomplete. For further depletion of the metal impurities, it is advantageous to slurry the catalyst in a nitric acid / hydrochloric acid mixture (aqua regia). In addition to platinum, all disruptive metals are also removed from the carrier. After separation of the carrier, e.g. B. graphite, the aqua regia solution is neutralized and a selective precipitant is added. The almost platinum-free precipitate is separated off and can be discarded or further processed. The carrier can be resuspended in the cleaned platinum solution and the noble metal can be noticed in the usual way. Alternatively, new carrier material can also be used.

If an exchange of the carrier material is planned anyway, the selective metal precipitation can also be carried out immediately after the old catalyst has been slurried in aqua regia and the suspension has been neutralized. The carrier material and the precipitation product can then be separated off and discarded.

A number of organic complexing agents have proven to be selective precipitation reagents which form complexes which are insoluble or poorly soluble in water with the metallic impurities. Particularly suitable are 8-hydroxyquinoline, quinosol, 5,7-dibromo-8-hydroxyquinoline, quinaldine and 1,5-diphenylthiocarbazone. Alizarin and 1,2-aminohydroxy-benzene derivatives are also suitable. The dosage can be in stoichiometric amounts, according to the content of the interfering metals. The quantity requirement is mostly visible from the fact that the precipitation formation stops when the precipitation reagent is added after the stoichiometric quantity requirement is exceeded.

The advantages of the method are based on the fact that it can be carried out without any technical effort in the course of normal regeneration. The total life of the catalyst, i.e. the intervals between complete catalyst changes are extended considerably. The old catalytic converter can be refurbished immediately during operation.

example A. Cleaning the catalysts

• 1. 50 g of a catalyst used for several years are stirred in dilute HN0 ₃ solution at 80 ^{C for} 16 h. The catalyst is separated off and suspended in dilute aqua regia. The filtrate is neutralized with sodium carbonate (up to pH 6-7). A 3.5% 8-hydroxyquinoline acetate solution is metered in dropwise until no further precipitation occurs on further addition. The precipitate is separated off and the filtrate is combined with the aqua regia suspension of the pre-cleaned catalyst. After neutralization with soda at pH 5 and the addition of sodium acetate, tetravalent platinum is completely reduced to the divalent stage at 80 ° C. with 3% aqueous sodium dithionite solution, with conc. Formic acid reductively on graphite and washed after the reaction. The content of interfering metal ions was reduced by 50%.
• 2. 600 g of dry used catalyst is cleaned and regenerated as in Example 1.
• 3. 50 g of a used catalyst used for several years are stirred in dilute aqua regia at 80 ° C for 16 h. After the support material has been separated off, the filtrate is neutralized to pH 4 to 5 with sodium carbonate. A 10% aqueous quinosol solution is added dropwise until further precipitation no longer occurs. After the sparingly soluble precipitate has been separated off, the filtrate is combined with the carrier and diluted aqua regia is added. After three hours of stirring at 80 ° C, the catalyst suspension is adjusted to pH 5 with sodium carbonate and after adding
  Sodium acetate at 80 ° C with 3% aqueous sodium dithionite solution. Tetravalent platinum is completely reduced to divalent platinum. With conc. Formic acid, the precious metal is struck on the graphite support. The content of interfering metal ions is reduced by> 90%.
• 4. 50 g of a used catalyst used for several years are stirred in 50 ml of diluted aqua regia at 80 ° C for 16 h. By adding sodium carbonate, the pH of the suspension is adjusted to 6 and 3.5% oxyacetate solution is added. Graphite and precipitation precipitate are separated off and washed with a little water, which is added to the filtrate. In the combined filtrate, 49.5 g of fresh graphite are suspended, with 10 ml of conc. Acidified aqua regia and stirred at 80 ° C for 6 h. The further processing to the catalyst is carried out analogously to experiment 3. When the carrier is completely replaced, less than 5% by weight of the platinum remains on the precipitate and is therefore lost.
• 5/7 Analogous to experiment 4, 1,5-diphenylthiocarbazone (5), quinaldine (6 and 5,7-dibromo-8-hydroxyquinoline (7) are used as precipitants with equally good results. For alizarin and aminohydroxybenzenes, the platinum losses are about 5 up to 15% by weight.

B. Testing the catalysts

• 8. 4.8 g of the catalyst regenerated according to Example 1 are suspended in a laboratory glass reactor in 120 ml of 4.3 n H ₂ SO ₄ and at 40 ° C. with vigorous stirring (3500 rpm stirrer) at 8.4 l / h for 4 hours Mixed gas consisting of 35.4% NO and 64.6% H ₂ implemented. The subsequent balancing of the analysis data results in a space-time yield based on NO
  
  
  The yield of hydroxylammonium salt was
  
  
  The corresponding data for a regenerated but not oxine-purified catalyst are given for comparison. The space-time yield here is 1.029, the hydroxylammonium salt yield but only 0.729.
• 9. 200 g of catalyst from Example 2 are suspended in 5 I of 4.3 N H ₂ SO ₄ in a large laboratory reactor and
  at 40 ° C with vigorous stirring (1000-rpm paddle stirrer) with 255 l / h of mixed gas with the composition 34.0% NO and 66.0% H ₂ until the normality of the acid solution has dropped to about 0.4 to 0.5 . The product solution is then decanted off. After adding new sulfuric acid, this reaction is repeated 3 times. The results based on the hydroxylammonium yield are compared in Table 1 below with the corresponding values of a freshly prepared standard catalyst.
  
  The catalyst, regenerated without oxine treatment, already shows no sufficient activity after the first treatment.

• 10. The tests of the catalysts in experiments 3 to 5 are carried out as in experiment 6.
  

Claims (3)

1. A process for regenerating a noble metal catalyst, in particular a platinum catalyst, which is applied to a carrier and has become poisoned by metals as a result of being used for many years in the preparation of hydroxylammonium salts by catalytic reduction of nitric oxide with hydrogen in an aqueous mineral

acid, wherein the metallic impurities are dissolved using nitric acid or aqua regia, the solution is neutralized, the troublesome metals are precipitated from the neutralized solution by means of a selective precipitating reagent, and the purified platinum solution is reused for the preparation of the catalyst.

2. A process as claimed in claim 1, wherein the metallic impurities are separated selectively from the catalytically active metal using an organic complexing agent as the precipitating reagent.

3. A process as claimed in claim 1, wherein a quinoline, carbazone, aminohydroxybenzene or anthraquinone derivative is used.